The present invention relates generally to hydraulic mining particularly to an improved hydraulic mining tool and method of hydraulically mining unconsolidated mineral formations such as tar sands.
Recent technology has been developed which permits the recovery of subterranean mineral deposits by use of hydraulic mining techniques. Hydraulic mining techniques basically involve the use of a high velocity liquid stream discharged directly into the subterranean mineral deposit to dislodge minerals from their surrounding mineral bed. The freed minerals and the discharged liquid stream form a resultant slurry that may be pumped by conventional pumping apparatus upward to ground surface for subsequent processing by surface separation systems. As the slurry is removed from the mineral formation, a mining cavity, or void, is formed in the mineral bed which, dependent upon the size and type of the particular formation, may extend to 100 feet in diameter throughout the height of the mineral bed. Examples of such hydraulic mining tools are disclosed in U.S. Pat. No. 3,951,457 issued to Redford and U.S. Pat. No. 3,439,953 issued to Pfefferle and my U.S. Pat. No. 4,275,926, the disclosures of which are hereby incorporated herein by reference.
To date, such hydraulic mining techniques have been primarily utilized to recover minerals such as uranium, coal, or potash, which typically possess sufficient consolidation in their natural formation state so that the mining cavity or void is formed in the subterranean formation, the surrounding mineral bed remains in its stabilized consolidated condition, thereby defining a "clean" mining cavity. Thus, in such consolidated formations, the overburden is continuously supported by the consolidated mineral bed and the hydraulic mining tool may be freely rotated and vertically reciprocated within the borehole and mining cavity throughout the mining process. However, in the hydraulic mining of unconsolidated mineral formations where the overburden is also unconsolidated, unique mining problems exist, which to a great extent have rendered the existing hydraulic mining tool technologyp potentially commercially uneconomical because of the caving of nonmineral-bearing overburden into the cave-in and mixing with the ore, thus decreasing the value of mined materials brought to the surface.
In contrast to the above-mentioned consolidated mineral formations, unconsolidated formations such as tar sands, typically are non-uniform in composition and often fail to possess the necessary degree of integrity and stabilization to maintain a cap over the mining cavity during the hydraulic mining operation. Failure of the unconsolidated overburden formations and, in particular, barren overburden has a serious effect on the economics of subterranean mining when the overburden cavens in and mixes with the ore. The compressive forces generated by the weight of the unconsolidated overburden will cause a cave-in effect as the ore body is being mined. As the subjacent portions of the tar sand mineral bed are removed during the hydraulic mining process, the overburden compressive force balance within the mineral formation is disturbed which, due to only minimal cementation integrity between the individual overburden sand grains, often results in a "cave-in" or "compaction" whereby the surrounding mineral bed catastrophically falls into the mining cavity and around the mining device.
When hydraulically mining in relatively shallow unconsolidated formations approximately 100 feet below ground surface, the removal of the mineral bed often permits the overburden to migrate downward into the borehole and the mining cavity, wherein it mixes with the mined mineral slurry and is subsequently transported upward to ground surface during the mining process. As will be recognized, the mining of the non-mineral bearing overburden reduces the overall efficiency of the mining process. Substantial mining of the overburden decreases the cost effectiveness of the hydraulic mining process to a degree such that the process is commercially infeasible. Further, in those instances where the downward migration of the overburden is acute, a general subsidence of the overburden may be experienced whereby the overburden fails to support the necessary surface mining equipment.
Alternatively, when mining in deep unconsolidated mineral formations (i.e., greater than 500 feet below ground surface), individual sand grains located proximate the borehole often dislodge from the mineral bed by frictional drag forces exerted by the rotating mining tool and drill string. Through prolonged duration, these frictional drag forces often disturb the fragile cementation forces existing between sand grains and result in the entire surrounding mineral bed falling in and compacting around the mining tool. Due to the depth at which the mining operation is occurring, substantial pressure is applied along the entire length of the mining tool. A partial collapse of the overburden could cause a canting of the mining tool, producing pressures on the sides thereof sufficient to prevent rotation of the mining tool, thus requiring intermittent shut-down of the drilling operation. In extreme instances collapses of the overburden have caused a complete structural failure or twist off of the mining tool within the formation. Such intermittent discontinuance of the mining operation significantly decreases overall operating efficiency while a twist-off condition typically results in the mining tool being irretrievably lost within the mineral formation.
The present inventor's copending U.S. patent application, Ser. No. 467,496, filed Feb. 18, 1983, and entitled APPARATUS AND METHOD OF HYDRAULICALLY MINING CONSOLIDATED MINERAL FORMATIONS, now abandoned, hereby incorporated by reference herein, discloses a hydraulic mining tool that includes means for injecting a suitable liquid bonding agent radially outward in the overburden formation from a borehole. The bonding agent cures to form a generally disc-shaped stabilized zone of overburden around the borehole above the mining cavity. The stabilized zone forms a rigid platform that artificially increases the cementation forces existing between the individual particles in the overburden, thereby increasing the suport of the overburden and preventing downward migration of the overburden during the mining operations. However, experience has shown that efficient hydraulic mining of some mineral deposits, particularly certain tar sands deposits, requires even more support of the overburden than is ordinarily possible by injecting a bonding agent from the mining tool radially outward into the mineral formation.
Thus, there exists a substantial need in the art for an improved hydraulic mining method and apparatus specifically adapted for use in unconsolidated mineral formations to prevent downward migration of the overburden, reduce frictional drag forces exerted on the mineral bed, and prevent generation of extremely high torsional forces on the mining tool during the hydraulic mining operation.